Thermoplastics have been widely used in the formation of medical parts for which excellent design, performance, and volume capabilities are required. For example, the use of plastics in the formation of packaging for physiologic solutions is well known in the medical field. They are usable whenever transparent and flexible packages, such as infusion-solution bags and blood bags, are needed.
In order to be useful in medical applications, a thermoplastic must meet various criteria. The plastics intended for packaging physiologic solutions must be inexpensive and easy to manufacture, must not react with the components of physiologic solutions, and must not leach additives into physiologic solutions. Their permeability to water-vapor, air and other gases must be low, their resistance to heat, radiation and ethylene oxide must be high since they are subjected to sterilization. Medical plastics also must be sealable by well-established methods (e.g., by radio frequency ("RF"), ultrasonics or the like), and they must withstand mechanical stresses common to medical environments.
One resin that has been used successfully in medical environments is poly(vinyl chloride) ("PVC"). The relevant art prior to and the subsequent development of this material as a medical plastic is reviewed in R. N. Burgess, ed., Manufacture and Processing of PVC, Macmillan Publishing Co., Inc., New York, 1982.
While PVC provides significant benefits, it is subject to certain drawbacks and disadvantages. Most notably, PVC requires a considerable proportion of plasticizers in order to make the material flexible and aid sealability. The tendency of the softeners and plasticizers to diffuse, leach or migrate from medical packaging made from such materials naturally limits their application.
Another deficiency of PVC is its tendency, under cold conditions common to storing physiologic materials, to loose flexibility and impact resistance. This can result in an undesirably high proportion of package breakages during transit and distribution. Also, infusion-solution bags made from softened PVC are often susceptible to degradation by atmospheric oxygen and microorganisms which can lead to additional package breakages.
Prior to the present invention, alternatives to PVC for medical packaging have been developed and used with varying degrees of success. Japanese Patent 86-002096 describes a packaging material consisting of foam prepared from a mixture containing an ethylene-vinyl acetate ("EVA") copolymer having 20-50 mole percent vinyl acetate and also containing an olefin resin such as polypropylene. The packaging material contains from 0.5-0.2 to 1 weight percent EVA.
U.K. Application 2,177,974 describes a packaging material which is a laminate having: (1) an inner layer of RF energy absorbent material such as EVA having a vinyl acetate content of at least 15%; (2) a first "heat-sealable" outer layer on one side of layer (1) comprising EVA having a vinyl acetate content of less than 15%; and (3) a second outer layer on the adjacent side of layer (1) which is suitably comprised of high-density polyethylene.
Canadian Patent 835,467 describes EVA compositions having improved thermal resistance which contain up to about 15% vinyl acetate and up to about 25% of (1) polypropylene; or (2) a mixture of polypropylene and high-density polyethylene. The compositions are suitable for medical applications such as packets for surgical instruments and containers for intravenous fluids.
There continues to be a need for cost effective alternatives to PVC that are useful in medical applications.